As already mentioned before, in this approximation the momentum transfer \(\vert \mathbf {q} \vert \)  is taken to be much smaller than both the mass \(m\)  of the heavy scalar and the energy \(E\)  of the massless particle, more precisely: \(\vert \mathbf {q}\vert \ll E \ll m\) . Following [1]}, [2]}, in the small-angle scattering limit the dominant four momentum transfer is in the transverse spatial direction, and one can assume that the amplitude obtained in this limit \(\mathcal {M(\mathbf {q})}\)  can be transformed to impact parameter space via a two-dimensional Fourier transform with respect to the momentum transfer \(\mathbf {q}\)  defined as:
\(i\tilde{\mathcal {M}}(\mathbf {b})=\frac{1}{4m E}\int \frac{d^{d-2}q}{(2\pi )^{d-2}}e^{i\mathbf {q}\cdot \mathbf {b}}i\mathcal {M}(\mathbf {q}) \ ,\) 
